1. Field of the Invention
The invention relates generally to method and apparatus for ion implantation applying mechanical scanning technology when a spinning batch type disk with multiple wafers (or substrates) moves laterally across a fixed ion beam. More particularly, this invention relates to an improved ion implanting apparatus and method to carry out large angle implantation with reliable angle control to reduce the tilt angle variation across the wafer for a batch type implanter.
2. Descriptions of the Prior Art
Conventional technology of batch type ion implanter is still limited by the technical difficulties of degraded angle integrity due to significant angle variations across the surface of a wafer in carrying out a large angle implantation. The implantation uniformity is degraded as the ion implantation process is carried out with a large tilt angle. Particularly, the difficulties exist in the batch type implanters wherein a rotating disk is implemented to carry multiple wafers, i.e. a batch of wafers. The disk is rotated and also can also be laterally moved across a fixed ion beam to expose the wafers to a uniform dose of the implanting ions over the whole batch of wafers carried on the disk. The system configurations of a batch type implanters were disclosed and described in several Patents, e.g., U.S. Pat. Nos. 5,350,427, 5,124,557, 5,218,209, 6,583,428, 6,593,699. A co-pending patent application Ser. No. 09/513,396 further discloses a batch type implanter that was filed by a common inventor of this Application and hereby incorporated herein as reference. The tilt angle variation of a batch size of 13 wafers across a 300 mm wafer is about xc2x11.2 degree if the wafer is flat, due to 5 degrees pedestal angle between wafer pad surface normal axis and the disk spin axis. The larger the pedestal angle, the greater is the tilt angle variation across a wafer. However, the pedestal angle of near 5 degree is necessary to keep wafers on the disk against the pads due to spin centrifugal force such that good contacts between the wafers and the pads can enhance the heat dissipation through thermal conductivity between the wafers and pads.
To minimize the tilt angle variation, the pad surface could be concaved, especially as cylindrical surface. In U.S. Pat. No. 6,222,196, Mack discloses an ion implanter including a rotatable support disposed in an implantation chamber of an ion beam implanter for supporting a plurality of wafer work-pieces. The rotatable support includes a hub adapted for rotating about an axis of rotation substantially parallel to a direction of an ion beam line entering the implantation chamber. The rotatable support further includes a plurality of wafer support members adapted to be attached to the hub, each wafer support member adapted to support at least one of the wafer work-pieces. Each wafer support member includes an attachment structure for affixing the support to the rotating member and a wafer support pad extending from the attachment structure and passing through the beam line as the hub rotates. The wafer support pad includes a wafer support surface facing the beam line that includes a concave portion and the concave portion of the wafer support surface is cylindrically shaped. The central axis of the concaved cylinder intersects an axis of rotation of the hub. The radius of curvature of the concave portion is large. For a pad to support a wafer with 300 mm diameter on a batch disk with 13 pads, the radius of curvature is approximately seven meters. Upon rotation of the hub at a predetermined angular velocity, the work-piece conforms to a shape of the concave portion due to a component of centrifugal force normal to a surface of the wafer support surface. However, the ion implanters as disclosed by Mack still have the limitation that the cylindrical surface of the wafer cannot resolve the difficulty of angle implant while maintaining sufficient wafer cooling during implantation. Although the support pad surface can be made cylindrical, it requires very large centrifugal force, or very high spin speed (as high as 2000 rpm), to deform a flat wafer onto cylindrical shape. It is impossible in reality for a batch disk to rotate at the high speed due to mechanical limitation, such as disk balance, disk material strength, and wafer damage by clamp mechanism. If a wafer cannot deform onto the cylindrical pad surface, its thermal contact with the water-cooled pad surface would be very weak and wafer cooling would not be sufficient. The problem of poor wafer cooling will be further discussed below.
The problems of angle variations across a large wafer surface are particularly pronounced when the semiconductor industry moves toward sub-micron node technology and the large angle implantation becomes a requirement for high dosage implantations. The conventional batch type disk cannot provide large angle implant due to large disk dimension in a narrow disk chamber. The largest tilt angle is 10 degrees provided by current commercially available batch type implanters. Two possible solutions are available to overcome these limitations when large angle implant is required using a batch type implanter. The first one is to tilt the whole disk together to an angle as large as 45 deg in a large process chamber. As that disclosed by Takeyama in U.S. Pat. No. 5,218,209 that an ion implanter, as shown in FIGS. 1A and 1B, for implanting ions into a batch of semiconductor wafers that includes a wafer holding disk of the centrifugal holding type with a plurality of wafer rests in the wafer holding disk having a wafer holding surface, which is conically curved. When the wafer holding disk is rotated, the wafer is pushed onto the wafer holding surface so that the surface of the wafer is curved nearly in the same manner as the conically curved inner surface of the peripheral portion of the s wafer holding disk. As a result, an ion beam being irradiated upon the surface of the wafer is kept at a perpendicular direction to the surface of the wafer. Takeyama""s method has several drawbacks. Specifically, the configuration enlarges the machine footprint. Furthermore, the beam-to-wafer distance varies at different tilt angle that makes it hard to control beam size at the wafer thus the beam current performance will be greatly reduced at low energy due to beam blow-up. Similar to Mack""s invention, this implanter also has a limitation and difficult that there is a poor contact between the wafers and the conical pad surfaces at practical spin speed of  less than 1000 rpm. The heat dissipation of the wafers can be a limitation for the implanter utilizing conically shaped wafer pad surface as that disclosed by Takeyama.
The other method is to tilt each wafer pad on the disk individually to a large angle as that disclosed by Aitken in U.S. Pat. No. 5,124,557. As shown in FIGS. 2A and 2B, Aitken""s method requires minimal disk tilt so that its process chamber thickness dimension requirement is similar to the wafer pad diameter. It has no impact on machine footprint, and no beam-to-wafer distance variation. Since each pad can rotate to horizontal position to allow horizontal wafer transfer on and off the pad, which results in better reliability than the vertical wafer transfers. However, the methods and apparatuses as disclosed by Aitken require the individual pad rotation control to vary the implant angle, thus introduces many technical difficulties that have not been resolved by the conventional implanters. Namely, the implanters are required to implement pad angle locking mechanism during fast disk spin, pad surface special design to minimize implant angle variation when a wafer on a tilted pad is passing through a doping ion beam during implant, pad rotation mechanism to overcome friction and mass inertia, and vacuum seal mechanism to separate rotating motors in air and substrate pads in vacuum.
For the above reasons, there are still needs exist in the art of ion implantation to provide improved method and apparatus such that the above-mentioned difficulties and limitations can be resolved.
It is the object of the present invention to provide a new system configuration and method for carrying out batch type ion implantations. The present invention discloses methods and special disk/pad configurations for a batch type implanter to apply large angle implant. The new disk/pad arrangements provide mechanism to rotate each pad to a large angle and lock it in place firmly against the large centrifugal torque trying to rotate the pad back to zero tilt angle, to minimize pad rotation error. Furthermore, this invention provides a method to concave into a wafer surface to follow a concaved pad surface similar to the wafer natural bending shape, with maximum deformation the same as the desired conical pad surface which gives minimal tilt/twist angle variation, to minimize tilt/twist angle variation during disk spinning and at the same time maximize thermal contact between the wafers and the concaved pad surfaces.
A preferred embodiment of this invention discloses a method to rotate individual pad of a batch disk to an implant angle and lock them in place, with the pad surface having a natural bending concaved surface to minimize the implant angle variation across a wafer on the pad for both tilt angle and twist angle, at large tilt angle implant. The implanter of this invention includes a disk with multiple attached pads that can hold substrates securely when the hub is at rest or rotates. The disk rotates around its spin axis, which moves laterally at a programmed speed profile so that all substrates on the hub can get evenly touched by the fixed ion beam. A counter weight is used to counter the centrifugal torque to keep the pad at any desired tilt angle while the disk spins at high speed.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed descriptions of the preferred embodiment that is illustrated in the various drawing figures.